Carbohydrate Catabolism by Fungi

CARBOHYDRATE CATABOLISM BY FUNGI D. GOTTLIEB Department of Plant Pathology, University of Illinois, Urbana, lllinois, U.S.A. To describe adequately the carbohydrate metabolism of the more than forty thousand species of fungi, from many diverse ecological habitats is, at this time, an impossible task. The morphology of the different species varies greatly; they range from relatively simple one-celled yeasts to the thread-like hyphal forms of the molds which ramify through and over the substrates, and to even more complex forms, such as mushrooms, which have large fruiting bodies. Despite the organization of cells into hyphae, mycelial mats, or reproductive organs, the fungi are essentially unicellular. Any single cell carries on all the basic metabolic functions and will give rise to all other morphological, communal, or reproductive forms of the species. Despite taxonomic sturlies of the large number of species, the biochemistry of very few üf these organisms has been studied. At best, one can describe the meta bolic activity of only those species that have been the object of research be cause of .)ome unique chemical characteristic, the~r economic importance or ease of handling. Even different members of the same species can vary greatly and the description of one strain of a species might not apply to other strains. N evertheless, we are being increasingly reassured of the funda mental similarity in metabolic activity among the fungi as the number of sturlies o:e these microbes steadily increases. The basic biochemical path way.; are those already described for most heterotrophic organisms that reqt:.ire m· can use exogenaus carbohydrate-either as an energy source or to furnish the carbon skeleton for other functional or structural metabolites. Only the hyphal fungi will be discussed in this paper, since the activities of the yeast jungi have been adequately described1. Fungi grow in or on a great number of living or non-living organic sub strates. Among these microbes are saprophytes, obligate parasites, and a large number of facultative parasites which can grow on either dead and living materials. The fungi are, therefore, confronted with a variety of carbohydrates, and the versatility of an organism in beingable to utilize a wide nurnber of such compounds could determine its chance of survival. Simple sugars, disaccharides, trisaccharides and glycosides of hexoses with other types of compounds are met; but probably the carbon sources in greatest abundance are the polymeric forms of simple sugars. These poly saccharides must first be hydrolysed to simple sugars before further meta bolism occurs. Most hydrolytic enzymes in fungi are constitutive, but some times they are adaptive, being synthesized only in the presence of the substrate. The fungi commonly contain sucrase or other ß-fructofuranosi• dases, though a few exceptions have been noted. This sugar is usually assumed w be hydrolysed outside the cell, but recent studies have shown 603 D. GOTTLIEB the Rhiz:,octonia solani hydrolyses the sucrase at the cell surface or inside the cell2• Spores of Myrothecium verrucaria, however, are believed to metabolize sucrase without prior hydrolysis3. Maltase is also ubiquitous among the fungi, though again a few exceptions have been reported. Galactosidase, cellobiase, trehalose melibiase also occur in many of these microbes. These enzymes need not be specific for each substrate, even though tri- and tetra saccharides can be used. Cochrane4 pointsout that probably many of these hydrolyses are carried on by more general enzymes, such as transfructisi dases, ß-glucosidases, or transglycosidases. Trisaccharides can thus be split wherever the appropriate linkages occur. Starchis hydrolysed by amylase, an enzyme which has been reported in almost all species. Wolf and Wolf5 have compiled a list of the enzymes found in 23 wood-inhabiting fungi, all ofthem produced amylase. Aspergillus oryzae and A. niger are used in the commercial production of these enzymes. Most data agree that mainly the ~-amylase is made by these microbes. Crystalline amylases have been obtained from Rhizopus delemar and A. niger. Cellulose is also utilized by many fungi which produce cellulase, ß-1,4- glucosidase, and hydrolyse this polymer to glucose. A complex of enzymes is probably involved. One of these is responsible for the swelling of cellulose without the appearance of free reducing groups-perhaps by attacking rela tively few points of attack along the chain. The other enzyme completes the hydrolysis into cellobiose or glucose units. Gellobiase is involved in final hydrolysis to simple hexose. A cellulase in Merulius lachrymans seems to split the cellulose molecule near its centre6, 7. The cellobiase is a hydrolase with transglycosidase properties. Experiments on Myrothecium verrucaria have given different results depending on the investigators: some hold to the con cept of multi-component whereas in others a single enzyme has been found. Enzymes für other types of glucose polymersarealso present. Laminarirr is hydrolysed by a ß-n-1,3 glucosidases. Trehalase, ß-1,2-glucosidase, and ß-galactosidases have been reported 9. Pentosans are also hydrolysed by a number offungilO, 11. Fungiare primarily aerobic organisms, nevertheless the anaerobic Embden MeyerhofPathway (E.M.P.) occurs in almost all species. The entire E.M.P. or parts of the system are even active under aerobic condition, but reduced oxygen concentration or the absence of oxygen favour the operation of this pathway. Many fungi produce ethanol and carbon dioxide, or often lactic acid12 and their formation from glucose strongly indicates an operative E.M.P. Among the many molds producing ethanol are A. niger, A. clavatus, A. glaucus, Mucor muceda and Rhizopus sp. That the E.M.P. is the pathway of alcohol production is shown by the 1 :1 ratio of carbon dioxide to ethanol that was produced by living cells of Fusarium lycopersici13 and F. lini14. Cell free preparations of A. nigeralso give similar results. Phycomycetes tend to produce lactic acid more commonly than do the higher fungi. The formation of both end products involve the E.M.P. and a reduction of acetaldehyde or pyruvic acid by reduced nicotinamide-adenine dinucleotide (NAD) and their respective dehydrogenases. The biochemical Iiterature on fungi con tains many examples ofindividual reaction product that involve the E.M.P.; M. plumbus and A. niger convert pyruvate to acetaldehyde. Many wood rotting Basidiomycetes convert ethanol to acetaldehyde15, 16, 604 CARBOHYDRATE CATABOLISM BY FUNGI The presence of the individual enzymes in the E.M.P. have been demon strated ;.n cell-free preparations for A. niger17, Penicillium chrysogenumlS and Claviceps purpurea19. They are present in spores of P. oxalicum, Ustilago maydis, Puccinia graminis tritici and Uromyces phaseoli20. Evidence for the participation of this pathway is also based on the dissimilation of 14C-labelled sugars. Under anaerobic conditions, glucose-1-14C is converted by F. lini to methyl labelled ethanol21. In other experiments with the same species, glucose- 1-14C was dissimilated with a recovery of only 1·8 per cent of the total activity in the carbon dioxide, but with glucose-3,414C the recovery was 53·1 per cent14. These results would be expected if the E.M.P. were used as the metabolic pathway. Under aerobic conditions, very slight conversion occurred via this system. Other evidence for the E.M.P. is the presence of the various intermediary products. P. chrysogenum mycelium contains glucose- 1-phosphate, glucose-6-phosphate, fructose-6-phosphate, fructose-1 ,6- diphosphate, adenosirre triphosphate (ATP) and nicotinamide-adenine dinucleotide phosphate (NADP). Studies with Iactate formation from glucose-l-14C gave the correct labeHing for the system, methyl-14C Iactate and smaller amounts of methyl-14C ethanol12. The aerobic, hexose monophosphate pathway (H.M.P.) is also wide spread in the fungi. Like the E.M.P., it converts glucose to metabolic inter mediates which can later be more fully oxidized in the energy producing cydes. This system is also a good source for the pentose needed in nucleotide synthesü.. Illustrative evidence for the operation of the H.M.P. is readily available. P. chrysogenum oxidizes C-1 of glucose more readily than the other carbons. Such data indicate the initial conversion of glucose to glucose-G-phosphate, 6-phosphogluconolactone, 6-phosphogluconate; then decarboxylation to carbon dioxide and ribulose-5-phosphate22. These results were confirmed by Heath and Koffier23. They demonstrated the participation of the Zwischenferment and NADP. Active participation of the H.l\1.P. was also demonstrated by radiorespirometry techniques in Ustilago maydis and Tilletia contraversa24. The individual enzymes of the entire system were shown 1n P. chrysogenum25, Claviceps purpurea19, Puccinia graminis tritici, Uromyces phaseoli, P. oxalicum and Ustilago maydis20. One should emphasize that under aerobic conditions, both the E.M.P. and H.l\1.P. may be active simultaneously. The quantitative röle of each system could depend on the age, on morphologic development ofthe fungus26 or even nutritive conditions. A survey of the data indicates that no one pa,:hway predominates in the fungi as a group. Increasing anaerobiosis tends to favour the involvement of the E.M.P., thus, though Verticillium alhoatrurr.: uses both pathways in air, only the E.M.P. operates under anaerobic conditions. Caldariomyces fumago does not seem to use the E.M.P. though it is likely that the system is present26. Less data is available on the r6le of Entner-Doudoroff pathway (E.D) 27. The usual initial phosphorylation of glucose is not needed. Gluconic acid is J:irst formed, then phosphorylated. The molecule is further oxidized to 2-keto-3 .. deoxy-6-phosphogluconate. Aldolase splits this intermediate to pyruvic acid and glyceraldehyde-3-phosphate. The pathway has been demonstrated in Tilletia caries spores24 and in germinating spores of C. fumago26. 605 2U D. GOTTLIEB In some fungi, glucose cannot be metabolized directly by the pathways mentioned above and must be transformed so that it can enter into these systems.

Load more